Hyperkalemic Periodic Paralysis

Hyperkalemic periodic paralysis (HyperPP) is an autosomal‑dominant channelopathy caused by pathogenic variants in the SCN4A gene, which encodes the skeletal‑muscle voltage‑gated sodium channel Nav1.4 [1]. The mutation produces a gain‑of‑function: channels fail to inactivate, allowing a persistent Na⁺ influx that depolarises the fibre membrane. This depolarisation opens voltage‑gated K⁺ channels, causing a rapid efflux of intracellular K⁺ and an acute rise in serum potassium. The membrane remains in a depolarised, inexcitable state, producing a brief episode of flaccid weakness. Between attacks the membrane repolarises, and many patients retain myotonia (delayed relaxation after voluntary contraction).